There are several kinds of stents on the market with either balloon expandable or self-expanding function. Balloon expandable stents are generally made from a material that can easily be plastically deformed into two directions. Before insertion, the stent is placed around the balloon section at the distal end of a catheter and pressed together to reduce the outer dimensions.
When the stent is delivered into the body in a desired location, it is expanded and thereby plastically deformed to a larger diameter by inflating the balloon. Once expanded, the stent supports the surrounding tissue and prevents at least local narrowing of the vessel.
Such plastically deformable stents need to have sufficient rigidity in the radial direction, but also some flexibility in the axial direction to enable delivery through tortuous anatomy. Furthermore, the amount of material should be as small as possible and the inner surface of the stent should not obstruct the flow through the channel (e.g., for blood) or cause too much turbulence.
Problems that generally occur after stent implantation are several: After crimping the stent onto the balloon of the delivery catheter, the stent experiences some elastic recoil to a slightly larger diameter, which can cause problems, e.g., snagging, when the catheter is advanced through the patient's vasculature. In addition, the engagement forces between the balloon and stent can become so small that the stent slips off the catheter. Moreover, a large stent delivery profile reduces the number of situations in which the stent can be used.
Another problem with balloon expandable stents is recoil of these stents after deployment. In this case, after expansion by the balloon of the delivery catheter, the stent outer diameter will shrink slightly once the balloon is deflated. The percentage change in deployed stent diameter due to recoil can be as much as 10%, and can cause migration of the stent.
A self-expanding stent typically is made of a more or less elastically expanding structure, which is affixed to the delivery catheter by some external means. For example, this type of stent is held in its constrained state by a delivery sheath that is removed at the moment of stent deployment, so that the stent self-expands to its preferred expanded form. Some of these stents are made of shape memory material with either superelastic behavior or temperature sensitive triggering of the expansion function.
A disadvantage of self-expanding stents is the need for the delivery sheath, thus resulting in a larger delivery profile. The removal of the sheath also requires a sheath retraction mechanism, which has to be activated at the proximal end.
Most balloon expandable and self expanding stents further have the disadvantage of that they experience large length changes during expansion and exhibit a poor hydrodynamic behavior because of the shape of the metal wires or struts.
Still further balloon expandable stents exhibit a positive spring rate, which means that further diametral expansion can only be achieved by higher balloon pressure. Moreover, previously-known stents typically are constructed so that external forces, working on the stent in the radial direction, may cause bending forces on the struts or wires of the structure.
For example, a unit cell of a Palmaz-Schatz stent, as produced by the Cordis division of Johnson & Johnson, or the ACT One Coronary stent, produced by Progressive Angioplasty Systems, Inc. have in their contracted delivery state a flat, rectangular shape and in their expanded condition a more or less diamond-shaped form with almost straight struts (Palmaz-Schatz) or more curved struts (ACT-One).
The shape of the unit cell of such stents is typically symmetrical with four struts each having the same cross section. In addition, the loading of the cell in the axial direction will typically cause an elastic or plastic deformation of all of the struts, resulting in an elongation of the unit cell in the axial direction. These unit cells have a positive spring rate. For stents based upon these unit cells, the stability against radial pressure is merely dependent on the bending strength of the struts and their connections.
In view of these drawbacks of previously known stents, it would be desirable to provide a stent having minimal elastic spring back upon being compressed onto a balloon catheter.
It also would be desirable to provide a stent having minimal recoil so that the stent remains at its selected deployed diameter after expansion.
It further would be desirable to provide a stent having a minimal length change during deployment of the stent.
It still further would be desirable to provide a stent that is not characterized by a positive spring rate, so that achieving further expansion does not require continually increasing balloon pressure.